Conventionally, an electric or hybrid vehicle comprises a traction system formed by high-voltage rechargeable batteries delivering a direct current to an inverter which transforms this direct current into an alternating current making it possible to power an electric motor, of the rotating electric machine type, the latter setting the vehicle in motion.
In order to recharge these high-voltage batteries, it is known practice to fit the vehicle with an on-board charging device essentially comprising an alternating-current—direct-current converter making it possible to rectify the alternating current power of the domestic electricity network into direct-current power for charging the batteries.
Usually, the charging device may also comprise a power factor corrector the role of which is to limit the harmonic rejection on the electricity network.
The electronic components of the power system on the one hand and of the charging system on the other hand are costly. Moreover, the powering of the motor and the charging of the batteries take place at different phases, so it has been proposed, in applications EP 0 603 778 and WO97/08009, to reuse a portion of the motor and of the components used for powering it in order to produce the device for charging the batteries.
Accordingly, the device for charging the batteries uses the inverter in order to form an alternating current—direct current converter and the coils of the motor to form the inductances. The transition from the motor powering mode to the battery charging mode is ensured by switching means with power contactors by disconnecting the neutral.
The use of the power connectors is however problematic because, ensuring the passage of the currents from the electric machine, they must be overdimensioned. In order to alleviate this drawback, one solution consists in producing a structure having switching means incorporated with H bridges.
However, in the abovementioned two cases, the use of the phases of the motor as an inductance in order to rectify the current of the electricity network causes disruptions in the rotor of the motor. Specifically, the inductances are magnetized by the alternating currents of the electricity network thus creating magnetic fields. These magnetic fields act on the rotor which may start moving, for example by vibrating, and even, depending on the magnetic fields and on the characteristics of the rotor, start to rotate. This setting in motion poses problems of both comfort and safety in the case of a use of the combined electric device in an electric vehicle even though the latter may be fitted with a system for decoupling the train of the machine during charging.
To alleviate this drawback, one solution consists in producing a static compensation consisting in injecting the charging current into at least one winding of the stator connected to a phase of the network by using an additional connection point, called a mid-point.
A winding of the stator conventionally comprises a plurality of coils formed by a plurality of turns.
The mid-point separates the phase winding of the stator into two portions so that the charging current, injected via the mid-point, is divided into two currents flowing in opposite directions through each half-winding, each half-winding comprising the same number of turns.
This solution of compensation by injecting charging current into the mid-points of the phase windings results in the magnetomotive force being cancelled out and the inductances of the two half-windings being cancelled out. All that remains apparent is the very weak leakage inductance associated with the imperfections of the coils.
Too weak an inductance of the motor makes it difficult to control the charging currents, notably because of considerably current inversions at the quench frequency.